The present invention relates generally to integrated circuit devices and more specifically to air-dielectric transmission lines used in systems for interconnecting transistors and integrated circuit devices.
As integrated circuits become smaller and wiring between transistors becomes more complex, the speed of the circuit becomes increasingly dependent on the metal interconnections between transistors and between integrated circuits. One problem that is particularly critical in high speed digital processors is inter-line capacitance between parallel interconnect lines. The inter-line capacitance found in bus architectures increases cross-talk, while the overall parasitic interconnect capacitance degrades circuit speed performance. Performance can, therefore, be improved by reducing the parasitic capacitance, particularly inter-line capacitance. One way to reduce inter-line capacitance is by using low dielectric constant materials in wiring interconnects.
Significant efforts are also going on in the research community to come up with a low dielectric constant material to replace the widely used silicon dioxide dielectric. Materials developments traditionally take a long time, however, since there are many reliability and manufacturing issues to be examined. As a result, it is advantageous to continue to use a well established materials system and obtain a reduction in cross-talk and parasitic loading by alternative structures.
While it is possible to reduce crosstalk by adding a ground plane or a coaxial shield, the addition of either a ground plane or coaxial shield adds capacitance thus reducing circuit speed. In addition, any added capacitance results in impermissible power dissipation in large chips due to the charging and discharging of the wiring capacitance. Attempts are being made to lower the voltage (V), but circuit frequency (f) is increasing so that power limitations from charging wire capacitance (C) tend to dominate current designs. Power dissipation in circuits is proportional to CV.sup.2 f.
Since air provides a low dielectric constant (effectively equal to 1) it is an ideal material for reducing inter-line capacitance. Air-dielectric transmission lines are well established in high-speed circuits using III-V group semiconductor materials. Two types of air-dielectric transmission lines in the prior art are the end-supported air bridge shown in FIG. 1 and the dielectric base air-dielectric shown in FIG. 2. The reference "Multilevel Microcoaxial Interconnect: A Novel Technology For VLSI Microwave Circuits" by M. E. Thomas et. al., 1991 VMIC Conference, pages 116-122, describes an end supported air bridge.
In the end-supported air bridge 110 shown in FIG. 1, the metal transmission line 112 is supported at both ends by a conducting cross-support structure 114a,b which is generally orthogonal to the transmission line 112. Since the middle portion 116 of the air bridge 110 is unsupported, the mechanical stability of the transmission line is suspect. Thus, conventional "end-supported" air bridges have not been accepted as feasible in high volume mainstream applications. Further, air is such a poor thermal conductor that heating of transmission line wires 112 is problematic.
FIG. 2 shows a perspective view of a dielectric-base air-dielectric transmission line 210. Unlike the end-supported air bridge 110 shown in FIG. 1, the metal transmission line structure 212 of the dielectric-base transmission line 210 is supported along the entire length by a dielectric base 214. Fabrication of such lines can be accomplished relatively easily for the top-most interconnect lines of integrated circuits by etching portions of the dielectric layer on which the transmission line structure 212 is supported using this structure as an etching mask. The transmission line structure is then surrounded on three sides by air and provides an apparent advantage over a line completely surrounded by a higher (non-air) dielectric. However, while the dielectric base transmission line 210 improves mechanical stability and thermal conductivity, the full width of the dielectric base simply redistributes and concentrates the charge to the point of highest dielectric constant, namely to the interface 216 between the conductive metal transmission line 212 and the dielectric base 214. The result is an improvement in interline capacitance, but only a small reduction in total parasitic capacitance.
The limitations of the dielectric-base air-dielectric transmission line 210 in the prior art are associated with the equality in width of the conducting line 212 and the dielectric base 214 in FIG. 2. To get the full improvement in total parasitic capacitance, the conductive line 212 would have to be almost fully surrounded by air because in that case the charge on the conductive line would be distributed nearly uniformly over its surface. The only exception occurs when the dielectric base 214 on which it rests has a height (h) many times larger than the dielectric width (w). However, the incorporation into an integrated circuit of a layer of dielectric of sufficient thickness to form such a dielectric base would pose an unacceptable manufacturing constraint.
A new approach to fabricating multilevel interconnects is called "damascene" after the inlaid jewelry method. Dual damascene refers to the method of making conducting via connections and conducting transmission lines by respectively etching holes and trenches in a dielectric film with two successive masking operations and filling them with metal. The reference "A four-level metal fully planarized interconnect technology for dense high performance logic and SRAM applications" by R. R. Uttecht and R. M. Geffken, 1991 VMIC Conference, pages 20-26 describes the dual damascene process. The dielectric in these structures fully surrounds the transmission lines just as in conventionally fabricated interconnects. Because the need for lower dielectric constants is so important and manufacturing issues in dual damascene still need to be resolved, research often is directed at new materials such as copper for conductors and polyimide for dielectrics. Simultaneous incorporation of new materials and new structures will make wide acceptance a slow process.
A transmission line which provides a mechanically stable structure, ensures acceptable heat dissipation, and provides low inter-line capacitance and interconnect parasitic loading that can be manufactured with existing materials systems is needed.